The number of total joint arthroplasty procedures performed worldwide has steadily increased over the past ten years. In the United States alone, nearly one million total knee and total hip replacements are performed annually. With no known biological remedies for osteoarthritis or rheumatoid arthritis, and with the U.S. elderly population expected to increase by about 7% by 2030, the number of total knee and hip arthroplasties is likely to increase.
In addition to these “primary” procedures, nearly 80,000 total joint devices fail and are revised every year. Recent data show that the number of revision surgeries has increased at about the same rate as primary surgeries, and the majority of revisions are due to total mechanical failure of the polymer bearing or patient reaction to polymer wear debris.
Ultra high molecular weight polyethylene (UHMWPE) is the material of choice for joint devices because of its toughness, low friction, and exceptional abrasion resistance. Due to its very high molecular weight, UHMWPE has a melt viscosity in the region of 1010 Pa·s and does not become fluid when heated above its crystalline melting point (138-142° C.). Instead, UHMWPE becomes a translucent, amorphous, rubbery material, which has limited consolidation of the resin to the use of methods, such as compression molding, direct compression molding, hot isostatic pressing, and ram extrusion.
Compression molding involves placing preformed or granular plastic material in a heated mold until it is softened to a pliable state. A hydraulic press then compresses the pliable plastic against the mold to form the desired object, and static pressure is maintained while the plastic cures. Ram extrusion involves heating plastic pellets or chips until the polymer is molten, then forcing the molten material through a die, and cooling the formed object. Typical pressures and temperatures for both compression molding and ram extrusion of UHMWPE are 3-5 MPa and 180-220° C. Processing at these pressures and temperatures leads to a small amount of autocrosslinking.
Outside the field of plastics, ram extrusion has been used to consolidate metal powders via a cold pressing process. This process involves applying pressure to a powdered metal sample in order to create a monolith. The consolidation process compacts the powder and causes the particles to be deformed such that the particles interlock and bind together. The deformed particles fill what would otherwise be void spaces, and the density and strength of the material increases.
Equal Channel Angular Extrusion (ECAE) is a specialized form of cold pressing. By forcing metal powder around a corner within a die, ECAE provides for the creation of bulk nanocrystalline material via low-temperature consolidation with no reduction in cross-sectional area, no introduction of impurities, and the possibility of a continuous—rather than a batch—process. The ECAE process has been successfully applied to metallic and ceramic materials; polymers, however, have not been consolidated through ECAE.